1. Field of the Invention
This invention relates to an apparatus for generating video signals and television cameras used in close circuit television for a monitoring system.
2. Description of the Prior Art
In a monitoring television system used as one of information transmission systems, at least one television monitor is connected through a selector to a plurality of television cameras via a plurality of transmission lines. Picture images picked up by one or several TV cameras are displayed on the TV monitor through the selector that employs an electronic switch. The electronic switch provides selection of any individual television camera or sequentially switches a plurality of television cameras for displaying a picture image of one camera after another on the television monitor or for recording into a video recorder.
In such an information transmission system it is preferable to mutually lock the internal synchronizing signals of a plurality of television cameras and the switching time to an external synchronizing signal in order to prevent the picture image on the monitor from being disturbed during and immediately after the switching operation from one television camera to another.
As devices for synchronizing a plurality of television cameras, there are known apparatuses for transmitting a vertical synchronizing signal or a vertical synchronizing signal and a horizontal synchronizing signal, or a composite synchronizing signal used in a television system.
In any of the known devices of the type under discussion, as the transmitted synchronizing signal itself is a train of pulses, which can be easily influenced by noise, the transmission of a synchronizing signal requires the use of coaxial cables with their high shielding effect or fiber optic cables, which makes it costly for the systems with a plurality of television cameras.
Another known apparatus for synchronizing a plurality of television cameras, is an apparatus for transmitting an external synchronizing signal from an external synchronizing generator to the television cameras by injecting the external synchronizing signal into the video signal transmission line and locking an internal synchronizing signal generator of the television camera by means of the transmitted external synchronizing signal. Such an apparatus is disclosed in U.S. Pat. No. 4,603,352, the entire contents of which are incorporated herein by reference.
Furthermore, in such an information transmission system, a selector which is disposed at the monitor site is connected to a plurality of coaxial cables or other transmission lines such as twisted pair, fiber optics for selectively receiving video signals. If several cameras of such a system are installed apart, from near and up to distant locations, the plurality of coaxial cables and/or other transmissions lines with random stretches of length cause different time delays for the propagation of the external sync toward the cameras and for the return propagation of the video signals toward the receiving end, and this causes synchronizing timing errors at the receiving end and thereby loss of synchronization.
It is an object of the present invention to provide a method and an apparatus for adjusting a time delay circuit associated with each of the television cameras and its transmission line to delay the application of external sync pulses to the internal sync generators of the cameras commensurating with the length of each transmission line, so that the timing of the internal sync of each television camera is adjusted to coincide with the timing of the external synchronizing pulse and the internal sync of all the other cameras at the receiving end, to thereby, overcome random timing errors.
Further, as the television cameras are scattered into distant locations the individual adjustment of each camera at its site is time consuming, another object of the present invention is to provide a method and an apparatus for remotely adjusting a time delay circuit associated with each of the television camera and its transmission line.
Yet another object of the present invention is to provide a method and an apparatus for an automatic self adjustment of a time delay circuit associated with each of the television cameras and its transmission line in a closed loop process between the cameras and a receiver.
According to the present invention an external synchronizing signal injected into the transmission line leading from the receiver to the transmitters or television cameras is modified in the receiver so as to produce a fluctuated external synchronizing signal.
The term xe2x80x9cfluctuated external synchronizing signalxe2x80x9d which is used throughout the present application means a broader term for changing identifying a synchronizing signal that includes changing of an amplitude or level of the synchronizing pulse and/or its width and/or its polarity and/or removal of one or a number of pulses from the pulse train, thereby altering or fluctuating the external synchronizing signal which is recognized by those video transmitters or television cameras which need to be adjusted.
The terms xe2x80x9ctransmitterxe2x80x9d and xe2x80x9ctelevision camerasxe2x80x9d used throughout the application means a video signal generating apparatus, analog or digital, incorporating an internal sync generator that can be synchronized or locked to an external synchronizing signal for outputting an externally synchronized composite video signal or a digital video signal.
The term xe2x80x9ctransmission line(s)xe2x80x9d used throughout the application means physically connected transmission lines such as coaxial cables or twisted pairs or optical fibers. It also means wireless communications such as a micro wave link, or a satellite link, or a combination of any of the above, being interconnected or routed via trunks, a public telephone network or Local Area Network (LAN) that provide two way communications between a transmitter and a receiver.
The above and other objects of the invention are attained by a method for a remote adjustment of time delay circuits of a plurality of television cameras, which method comprises the steps of transmitting an external synchronizing signal from a receiver to said television cameras through a plurality of transmission lines, and receiving a composite video signal from a selected television camera among said plurality of television cameras through a selected transmission line for displaying said received composite video signal on a monitor, and for identifying a timing error between the timing of said external synchronizing signal transmitted from said receiver to said selected television camera and the timing of the sync portion of the received composite video signal from said television camera, wherein said external synchronizing signal is modified in said receiver to produce a fluctuated external synchronizing signal whenever said receiver identifies a timing error and wherein a delay circuit in said television camera will gradually adjust a time delay thereof when it identifies that the external synchronizing signal is fluctuated or stop a time delay adjustment thereof when the external synchronizing signal is no longer fluctuating.
The objects of the present invention are further attained by a remote adjustment of a time delay apparatus for a television system which comprises a plurality of transmitters for transmitting a composite signal or a digital video signal; a receiver for receiving said composite signal or a digital video signal; and a plurality of transmission lines for transmitting and receiving information between said transmitters and said receiver, said receiver including a generating circuit for generating an external synchronizing signal, a transmitting circuit for transmitting said external synchronizing signal to said transmitters through said transmission lines, a detecting circuit for detecting time errors between the time of said external synchronizing signal and time of the sync portion of said received composite video signal and for outputting time error signal, and means for modifying the external synchronizing signal for producing a fluctuated external synchronizing signal when a time error signal is received, each of said transmitters including a detecting circuit for detecting said external synchronizing signal for applying the detected synchronizing signal to an internal sync generator to lock said internal sync generator to said external synchronizing signal, an adjustable timer circuit for delaying an application of said detected synchronizing signal, a sensor circuit for detecting a fluctuated external synchronization signal and feeding a fluctuated signal to said adjustable timer for adjusting the delay time.
In a preferred embodiment of the present invention, the circuits for fluctuating the external synchronizing signal and for detecting the fluctuated synchronizing signal are disclosed in U.S. Pat. No. 5,335,014, the entire contents of which are incorporated herein by reference.
The external synchronizing generator may include a circuit for generating external synchronizing pulse signals having a level higher than the white level or lower than the black level of the composite video signal generated by the transmitters, injecting the synchronizing pulse into the video transmission lines connected to the transmitters, and synchronizing the internal synchronizing generators of the transmitters on the basis of the injected synchronizing pulses.
The transmitters preferably each include a level comparator circuit for separating the external synchronizing pulse signals from the composite video signal on the basis of a level difference and applying the separated external synchronizing pulses to the transmitters.
According to the present invention, as the transmitters are synchronized by a pulse signal propagated from the receiver through the video transmission line and the propagation time delay is adjusted by fluctuating said pulse signal the adjustment of the time delay circuit can be carried remotely from the receiving end.
The foregoing and other objects and features of the invention will become apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.